As regards the state of the technique, the converters are of the following types. Certain of these are known as precise load compensation converters, some of these being relatively rapid, or those known as "flash" converters, which are extremely rapid but less precise. The load compensation converters may be precise double ramp converters providing a measuring result on n points at the end of a period 2n times the cycle time of a counting clock contained in these converters. They may be frequency-voltage or frequency-current converters as precise as and twice faster than double ramp converters. They may also be delta modulation converters. All the converters may be preceded by a blocker-sampler so as to memorize the value of the signal over an extremely short period of time and then convert it. Furthermore, it is possible to add to the input of all the converters a Vo polarization signal so as to limit the dynamics from Vo to Vo, plus the signal to be converted.
All the load compensation converters comprise an automatic control or threshold serving to compensate the loads. In all these converters, the signal is either a current or a voltage transformed into a current by a resistor. The total load brought by this current is measured for a given time To. The principle is always the same. The load is accumulated in a capacitor of an integrator and from this capacitor loads of known values are withdrawn, said values being identical or different, withdrawal being effected either continuously or by load packs, or during the time To or after this time To, or during and after this time To. In any event, this amounts to withdrawing loads of known values so as to have at the end of a certain time a nil load in the accumulation capacitor. Thus, the total load brought by the signal to be converted has been transformed into a known number of loads. This number represents a digital measurement of the signal.
In a double ramp converter, loads accumulate for a period To in the capacitor and are continuously withdrawn after To. The discharge time is representative of the measurement of the signal. In a frequency-voltage or frequency-current converter, constant discrete loads are withdrawn during To so that the voltage at the terminals of the capacitor remains extremely low. The extraction frequency of the loads is representative of the measurement of the signal. In delta modulation converters, discrete loads of known identical positive or negative values are fixed frequency injected. The sign of each load is selected so that the voltage at the terminals of the capacitor remains low. These loads are injected during the accumulation time To. The difference between the number of positive loads and the number of negative loads is representative of the measurement of the signal.
There now follows a more precise description of the particular principle of the frequency-voltage or frequency-current converter. This consists of using an analog integrator receiving on an input the sum of the signal to be digitalized, in other words to be quantified, and a certain number of quantization loads of identical values, the polarity of said loads being inverse to that of the signal to be quantified. The average flowrate of the quantization loads is proportional, at an almost residual load, to the amplitude of the signal to be quantified, the injection frequency of these loads being variable and depending on this amplitude.
Counting of the number of the quantified electric loads injected at the input of the integrator is effected as follows. The output of the integrator is connected to the input of a voltage threshold comparator. The output of the threshold comparator is connected to a quantified load generator. When the signal to be quantified is introduced into the integrator, the integrator starts to integrate this signal: the output signal of this integrator evolves with the time involved. When the output of the integrator reaches the threshold of the threshold comparator, this comparator flips and a quantified load is injected at the input of the integrator so as to cause to abruptly vary in the opposite direction the output signal of this integrator. However, the signal to be quantified continues to be introduced at the input of the integrator so that the output signal of the integrator starts again evolving and so that this cycle is reproduced. It is important to mention that this cycle is reproduced at a rhythm which is all the faster when the signal to be quantified itself is high. Thus, the quantization principle is simple: for a given period, a count is made of the number of times it has been necessary to inject quantified electric charges into the integrator. Thus, for example, for a counting period of about one millisecond and by using an injection-integration chain able to admit a cycle of 10 MHz, it is possible, for a maximum amplitude signal, to count up to ten thousand injected loads. This corresponds to a ten thousand points measuring converter. In binary mode, this is roughly equivalent to 14 high order bits.
However, in certain applications and in particular in the envisaged tomodensitometer application, these quantization dynamics are insufficient. In effect, it is necessary to provide a quantization with more precision, for example with from one to one hundred million measurement points. In order to increase precision of the measurement, the fact is used of knowing that at the end of a given period allocated to drawing up a digital value of the signal to be quantified from the number of charges injected into the integrator, the latter retains a non-nil residual charge. This charge is due to integration of the signal during the integration period which succeeded the last injection and which was extended until the end of the given period which corresponds to the end of integration. Finally, quantization of this charge or load makes it possible to increase the dynamics of the converter. In the remainder of this text, the digitalization values obtained from the number of charges injected into the integrator shall be called high order values, and the digitalization values obtained from the residual charge shall be called low order values.
The quantization principle relating to low order values is thus the following: the integrator is discharged by connecting its input to a reference current generator. The period is measured at the end of which the output of this integrator returns to zero. This period is representative of the low order values of the signal. In order to know these values, it is merely sufficient to have function, at the rhythm of a constant step clock, a counter for the period which extends from the start of the discharge until the moment when the output of the integrator returns to zero. Thus, the drawing up of the high order values is effected by a repeated discharge of the integrator, whereas the drawing up of the low order values is effected by measuring the constant current discharge time of the residual charges contained in this integrator at the end of drawing up the high order values. A converter taking account of the residual charge in order to digitalize a signal is described in an article entitled "RESOLVE 22 BITS EASILY WITH CHARGE BALANCE ADCs" by Mr. Thomas J. MEGO and published in the journal called ELECTRONIC DESIGN and dated the 25th June 1987. This converter carries out a charge equipoising type conversion in order to determine the high order values of digitalization. In this converter and for drawing up high order values, a current Io is injected at the input of the converter so as to limit the dynamics to (Ix+Io)/Io, Ix representing the input signal of the converter to be digitalized. Thus, the dynamics are sufficiently reduced so as to use two standard charge values Q/8 and 5 Q/8. These charges are drawn up from a standard resistor R connected between the input of the integrator and a discharge reference potential source Vref by means of a swicth controlled by variable duration voltage pulses. The charge Q/8 is continuously injected and at a constant frequency. It serves to compensate the current Io. As soon as the stored charge becomes too large, the charges Q/8 are replaced by charges 5 Q/8. This stratagem makes it possible to inject at a fixed frequency a charge Q/8 or 5 Q/8 according to the charge equilibrium requirements. The measurement of the number of charges injected provides a high order measurement of the signal. Moreover, in order to draw up the low order values, the residual charge is discharged through another standard resistor connected by means of a switch to the reference potential source, the low order values then being determined from the discharge time.
This type of converter comprises a certain number of drawbacks. In the case of drawing up low order values, the switch being controlled by a fixed frequency discharge oscillator on each voltage pulse, the switch is temporarily closed and the capacitor of the integrator runs down from Q=Vref. T/R. The output voltage variation Vs of the integrator corresponding to this discharge depends on the time constant which the capacitor forms with a standard resistor of the amplitude Vref of the discharge reference potential and the period T during which on each pulse the switch is closed (Vs=Vref. T/R.C).
Now, controlling the closing period of the switch is a delicate problem. In effect, if the maximum conversion period is estimated at one millisecond and ten thousand precision points are required for the measurement, it is therefore necessary to have a pulse cycle of about 10 MHz. It is then easily possible to show that the discharge period, given the fact that it is repeated ten thousand times, shall be adjusted more precisely than the ten thousandth of its elementary period. This amount to stabilizing the conduction period of the switch with this same precision. The fluctuation of the conduction period shall be less than one tenth of a nanosecond. This is difficult to obtain. In the case of drawing up low order values, the residual charge residing in the capacitor of the integrator at the end of quantifying the high order values is low. In order to be able to measure it with sufficient precision, namely finally for a long enough period of time, it is necessary to discharge it with a low current. In order to do this, the capacitor is discharged by the corresponding standard resistor.
Having a regard to the charges which can be accumulated in the integration capacitor and its envisageable size in practice, discharge standard resistors have been selected, said resistors being of about one hundred GIGOHMS. These resistors are very expensive and in no instance may be integrated where this converter is integrated in an MOS type integrated circuit. Finally, the principle is this converter thus requires the precision control of several references: the conduction period of swicthes, the value of the standard resistors and the stability of the reference potential.